This invention relates to acidic shape selective zeolite catalysts which have been pretreated to enhance catalyst life and methods for their use to synthesize olefins, in particular by conversion of lower monohydric alcohols and/or their ether derivatives.
Olefins, especially ethylene and propylene, are used on a large scale as intermediates for the manufacture of staple products such as olefin polymers, ethylene oxide, non-ionic detergents, glycols and fiber-forming polyesters. Processes for producing olefins usually involve non-catalytic pyrolysis of volatile hydrocarbons such as natural gas liquids or petroleum distillates. Catalytic pyrolysis processes have been proposed but do not appear to have reached industrial use.
In countries where such volatile hydrocarbons are not accessible but such feedstocks as coal, oil shale and methane, and consequently carbon monoxide/hydrogen synthesis gas derived therefrom, are available, it would be desirable to produce olefins from synthesis gas. It has been proposed to do this by converting the synthesis gas to methanol or to hydrocarbons and/or their oxygenated derivatives and reacting such products over shape selective acidic zeolites, e.g., of the ZSM-5 family. (See for example U.S. Pat. Nos. 3,894,106; 4,025,571; and 4,052,479).
Shape selective zeolite materials, both natural and synthetic, have been demonstrated in the past to have catalytic capabilities for various types of organic compound conversions. These materials are ordered porous crystalline metasilicates (e.g. aluminosilicates) having a definite crystalline structure within which there are a large number of cavities and channels, which are precisely uniform in size. Since the dimensions of these pores are such as to accept, for adsorption, molecules of certain dimensions while rejecting those of larger dimensions, these materials are deemed to possess the property of shape selectivity, have been referred to as "molecular sieves", and are utilized in a variety of ways to take advantage of these properties.
Such shape selective molecular sieves include a wide variety of positive ion-containing crystalline aluminosilicates, both natural and synthetic. Aluminosilicates can be described as a rigid three-dimensional network of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen is 1:2. The electrovalence of the tetrahedra-containing aluminum is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation. This can be expressed by formula wherein the ratio of Al to the number of various cations, such as Ca/2, Sr/2, Na, K or Li is equal to unity. One type of cation may be exchanged either in entirety or partially by another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the size of the pores in a given aluminosilicate by suitable selection of the particular cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
A preferred group of shape selective crystalline aluminosilicates, designated as those of the ZSM-5 type (e.g. see U.S. Pat. No. 3,702,886) as well known for use in the synthesis of olefins from syn gas derived materials such as methanol. Other shape selective zeolite materials are also well known for this purpose as discussed in the aforedescribed patents. Characteristic of any shape selective catalyst capable of catalyzing the conversion of methanol and/or dimethyl ether to higher hydrocarbons is its possession of acid sites which are believed to be responsible for catalytic activity. The acidity characteristics of such shape selective zeolites are believed to be related to the amount of alumina in the zeolite (see U.S. Pat. No. 3,941,871), e.g., it is generally recognized that the lower the alumina content of the zeolite the lower the overall relative acidity of the zeolite.
Unfortunately, the use of acidic shape selective zeolites to catalyze methanol and/or dimethyl ether conversion for olefin production is not entirely satisfactory because such zeolites are also well known to catalyze the formation of higher hydrocarbons such as C.sub.5 + paraffins, aromatics and alkylated aromatics. The particular distribution of products obtained from the use of any given catalyst is typically controlled by the reaction conditions, particularly temperature. Thus, while there is not a clear line of demarcation in product distribution as a function of temperature, it has been recognized (for example, see U.S. Pat. No. 3,894,107) that as the reaction temperature is increased, the methanol conversion can be shifted in favor of the formation of ethers, olefins, aromatics and alkylated aromatics at respectively higher reaction temperatures. The use of temperature control to influence product distribution is illustrated in U.S. Pat. Nos. 4,052,479 and 4,058,576 wherein staging of the reactions is employed. The partial pressure of the reactant feed has also been observed to influence olefin selectivity. Thus, U.S. Pat. No. 4,025,576 discloses the use of a subatmospheric partial pressure of the reactant feed to improve its conversion with enhanced olefin selectivity. Subatmospheric partial pressure of the reactant feed is obtained either by maintaining a partial vacuum in the conversion zone, or by co-feeding a diluent. Suitable diluents include any substantially inert substance that is a gas or vapor at reaction temperature such as steam, as well as nitrogen, carbon dioxide, carbon monoxide, and the like. When such diluents are used, total pressure in the reaction zone may range from subatmospheric up to about 1500 psia depending on the amount of diluent introduced with the feed. The diluent serves to assist in removing the heat of reaction generated in the more exothermic alcohol or ether conversions. Typical reaction temperatures vary from 500.degree. to 1000.degree. F. (260.degree. to 537.degree. C.).
In addition to controlling reaction conditions, product distribution favoring olefin production can be influenced substantially by modifications in the acidic shape selective zeolite catalyst. Thus, various cations can be used for modification by exchange as illustrated by U.S. Pat. Nos. 4,079,096 and 4,066,714.
U.S. Pat. No. 3,911,041 discloses a phosphorus modified zeolite prepared by reacting the latter with a phosphorus containing compound having a covalent or ionic substituent capable of reacting or exchanging a hydrogen ion. The phosphorus containing zeolite is described as possessing a greater number of acid sites than the parent zeolite but these more numerous acid sites appear to have a lesser acid strength than those found in the unmodified zeolite. It is suggested that the replacement of the strong acid sites in the parent zeolite, with a greater number of relatively weak acid sites may be responsible for blocking the aromatizing activity of the unmodified zeolite. A further increase in the number of weak acid sites is said to be effected by contact of the zeolite with water vapor, preferably prior to use of the zeolite as a catalyst and subsequent to phosphorus modification. The location of the strong acid sites is unspecified. The phosphorus modified zeolite is alleged to enhance the selectivity to light olefins and decrease the selectivity to aromatics. Olefin selectivities (ethylene, propylene and butene) disclosed include 38.4% at a methanol conversion of 85% (Table 1, Run 2); and 43-70% at conversions greater than 75% (Table 4). At 300.degree. C. reaction temperature, and 0.22% of DME conversion, the olefin selectivity is 100% (Table 6). This selectivity drops as the temperature is increased to 350.degree. C. (% olefin selectivity 49.8 at a 5.7% DME conversion) and drops further as the reaction temperature increases to 400.degree. C. (% olefin selectivity 45.0% at 56.7% conversion) as also disclosed in Table 6. Table 10 discloses DME conversions ranging from 3.6 to 100% at olefin selectivities which vary between 12.7% and 73.4%.
Kikuchi et al suggest in the article "Acid Properties of ZSM-5 Type Zeolite and its Catalytic Activity in Methanol Conversion" J. Japan. Petrol. Inst. Vol. 25, No. 2, pp. 69-73 (1982) that poisoning the acid sites located at the external surface or around the entrance to the pores of a ZSM-5 type zeolite with 4-methyl quinoline inhibits aromatic hydrocarbon production from methanol, and conclude that such acid sites participate in the formation of aromatic compounds such as trimethyl benzene and tetramethyl benzene. Such poisoning with 4-methyl quinoline is a result of neutralization of acid sites with a base and is not due to coke formation.
European Patent Application Publication No. 54,375 discloses a process for converting a methanol containing feed to hydrocarbons including olefins using a shape selective zeolite such as the ZSM-5 type, both modified and unmodified, as well as dealuminized mordenites, and faujasite. A promoter is incorporated into the reactant feed to accelerate the conversion of methanol to hydrocarbons, particularly olefins, which permits the use of reaction temperatures less severe than those associated with prior art processes needed to achieve similar conversions, thereby leading to enhanced olefin selectivity attributable to the use of such lower reaction temperatures (page 7, lines 2 et seq). Suitable promoters include aromatic hydrocarbons and their precursors, olefins and their precursors, and aldehydes, e.g., formaldehyde. In the case of ZSM-5 type zeolites the Al content is varied from 0 to 4%, the lower limit to Al content being embodied in a composition referred to as silicalite, e.g., SiO.sub.2 :Al.sub.2 O.sub.3 mole ratio of 1600:1.
Notwithstanding the prior art processes for enhancing olefin selectivity, such methods inevitably are accompanied by complex side reactions such as aromatization, polymerization, alkylation and the like to varying degrees. As a result of these complex reactions, a carbonaceous deposit is laid down on the catalyst which is referred to by petroleum engineers as "coke". The deposit of coke on the catalyst tends to seriously impair the catalyst efficiency for the principal reaction desired, and to substantially decrease the rate of conversion and/or the selectivity of the process. Thus, it is common to remove the catalyst from the reaction zone after coke has been deposited thereon and to regenerate it by burning the coke in a stream of oxidizing gas. The regenerated catalyst is returned to the conversion stage of the process cycle. The period of use between catalyst regenerations is often referred to as catalyst life. There are obvious economic incentives to improve the catalyst life such as the savings in capital investment to achieve regeneration, such as for example, the installation of a fluid bed reactor system, or a multiple bed reactor system. Furthermore, a rapidly deactivating catalyst produces a product distribution which changes substantially with time, thereby complicating the downstream purification operation.
It has been observed (see U.S. Pat. No. 3,941,871) that while lowering the aluminum content of a shape selective zeolite will reduce coke formation, excessive reduction in the alumina content will destroy the activity of the zeolite catalyst responsible for olefin production.
An alternative approach to coping with the coke formation problem is to employ a fluidized bed wherein the coked catalyst can be continuously regenerated. At least one fluidized system intentionally maintains a high coke level between about 5 and 20% by weight of the catalyst, as disclosed in U.S. Pat. No. 4,328,384. This patent discloses the use of a fluidized bed riser reaction scheme for the production of gasoline range boiling products, wherein the catalyst particles are segregated into a lower disperse fluidized phase, and an upper dense fluidized phase. The methanol feed is converted to dimethyl ether and olefins in the lower disperse catalyst phase. The olefins from the lower disperse phase are then converted to gasoline range boiling products in the upper dense phase. This reaction scheme is employed to provide minimum contact of the methanol with the final desired products, particularly aromatics, since methanol and aromatics react to form tetramethyl benzene (durene), an undesirable component in gasoline. The catalyst employed in this reactor scheme is regenerated under conditions sufficient to achieve only a partial removal of coke therefrom rather than provide a clean, burned catalyst. Maintaining a high coke level on the catalyst in the range of 5 to 20 weight percent, reduces the catalyst activity, and olefins are preferentially produced for a given space velocity under selected temperature conditions. While methanol conversions up to about 70% are alleged to be obtainable in this manner (Col. 12, line 32) only about 7% of the product from the disperse phase comprises C.sub.2 to C.sub.5 olefins (Col. 13, line 66). Lowering the coke level during regeneration increases catalyst activity, but at the expense of C.sub.2 to C.sub.5 olefin selectivity. The catalyst is withdrawn from the dense upper phase for regeneration and it can be transported to the regeneration zone with an inert gas or with the regeneration gas of desired O.sub.2 concentration (Col. 13, lines 25 et seq). The regenerated catalyst is introduced into the upper dense phase from which is withdrawn catalyst that is introduced into the disperse lower phase for olefin production. Thus, the coke content of the catalyst when it enters the disperse phase is even higher than the freshly regenerated catalyst due to the additional coke deposits which form while the catalyst is in the high aromatics containing dense phase prior to removal therefrom and introduction into the lower disperse phase. The use of such excessive amounts of coke on the olefin producing catalyst is believed to be responsible for such lower catalyst activity.
Precoking of a zeolite catalyst used for the alkylation of aromatic compounds is disclosed in U.S. Pat. No. 4,276,438 (Col. 13, lines 24 et seq). The effect of the precoking is undisclosed.
U.S. Pat. No. 4,229,608 is directed to a cyclic heat balanced process for converting methanol and/or dimethyl ether to a product rich in ethylene and propylene and containing less than 20 weight percent methane, using a fluidized zeolite catalyst at temperatures of between about 425.degree. and 760.degree. C. (e.g., 800.degree. to 1200.degree. F.). During the conversion reaction less than 9 (e.g., 4 to 6) weight percent of the feed is converted to a carbonaceous deposit (coke) on the zeolite amounting to between about 0.4 and 1 weight percent of the zeolite. The mixture of spent zeolite catalyst and hydrocarbon product are separated and the spent fluidized zeolite catalyst regenerated by combustion in air at temperatures of between about 650.degree. and 760.degree. C. and the carbonaceous deposit removed therefrom (see Col. 2, lines 11 et seq). At Col. 2, lines 18 et seq, it is noted that water generated as a by-product in the conversion reaction had been observed to cause a substantial loss of activity of crystalline aluminosilicate zeolites by steaming. The cyclic heat balanced operation disclosed in this patent is alleged to increase the stability of the zeolites employed therein. At Col. 2, lines 26 et seq, it is concluded that under the specified conditions of short contact time and controlled temperature, the hydrocarbon sorbed product and/or the residual carbonaceous deposit, remaining on the catalyst after regeneration, may serve to protect the catalytic sites of the zeolite. The amount, if any, of residual carbonaceous deposit on the zeolite allegedly remaining after regeneration is undisclosed. Furthermore, the regenerated catalyst is not subjected to a heat conditioning step prior to introduction back into the conversion zone.
U.S. Pat. No. 4,231,899 is directed to a method for producing a steam stable aluminosilicate zeolite catalyst. In accordance with this process a zeolite, containing organic cations and/or in contact with a charring agent is calcined at a temperature below 1100.degree. F. (593.degree. C.), e.g. 800.degree. to 1050.degree. F. (i.e. 426.degree. to 565.degree. C.) to convert a portion of the organic cations and/or charring agent to a carbonaceous material and deposit at least 1.5-15 wt.% thereof within the pores of the zeolite. Fresh uncoked catalyst is said to possess much more activity (e.g. excessive active sites) than is required for the conversion of oxygenated hydrocarbons. The precoking procedure of this patent is therefore asserted to protect these excess active sites from steam deactivation until such time that they can be revived during regeneration (Col. 2, Lines 36 et seq). It will be observed that the precoking procedure is conducted (in the presence of oxygen, Col. 8, Line 24, as contrasted to conventional techniques, Col. 10, Line 33) for 16 hours or more, and no heat treatment as defined herein is employed subsequent to the precoking procedure. It is further observed that the coke formed during precoking is deposited "within" the pores of the zeolite (Col. 8, Lines 55 et seq).
U.S. Pat. No. 4,358,395 is directed to a process for converting lower monohydric alcohols and ether derivatives thereof with a precoked crystalline aluminosilicate zeolite. Catalyst precoking is conducted by exposing the zeolite to a thermally decomposable organic compound at a temperature in excess of the decomposition at a temperature e.g. generally greater than 1000.degree. F. (538.degree. C.), but less than 1200.degree. F. (649.degree. C.) at a hydrogen to organic compound mole ratio between 0 and 1, to deposit at least 1% coke, predominantly on the surface of the zeolite (Col. 9, Lines 57 et seq). When precoking temperatures are less than 1100.degree. F. (593.degree. C.) at least 0.2 mole of H.sub.2 per mole of organic compound is employed (Col. 6, Lines 55 et seq). Suitable thermally decomposable organic compounds include paraffinic, cycloparaffinic, olefinic, cycloolefinic, and aromatic compounds, as well as oxygen containing compounds such as alcohols, aldehydes, ketones, ethers, and phenols. Preferably, the organic compound is the same as that subsequently undergoing conversion (Col. 7, Line 5). An additional optional treatment either before or after precoking involves contact of the catalyst with steam. This patent distinguishes the coke deposited pursuant to precoking (referred to as selectivity enhancing coke), and coke deposited during the hydrocarbon conversion reaction (referred to as activity reducing coke). During regeneration in a hydrogen containing atmosphere, the activity reducing coke is removed, while the selectivity enhancing coke is not. The precoked catalyst is alleged to be suitable for a wide variety of hydrocarbon conversion reactions, including the conversion of lower monohydric alcohols and ethers to olefins, although this reaction is not illustrated in the examples, the only reactions illustrated by example being limited to toluene disproportionation and toluene alkylation. It will be observed that precoking times illustrated in the examples range from 16 to 112 hours at precoking temperatures of no less than 1050.degree. F. (565.degree. C.). Furthermore, this patent fails to show a heat treatment as defined herein, subsequent to precoking.
In view of the above, there has been a continuing search for ways to improve catalyst life while still attaining acceptable, and most preferably, improved olefin selectivity and/or yield. The present invention was developed in response to this search. SUMMARY OF THE INVENTION
It has been found that the life and/or selectivity of acidic shape selective zeolites can be improved substantially when these catalysts are employed for the conversion of alcohols and/or ethers to lower olefins by a controlled hydrocarbon pretreatment procedure followed by further heat conditioning of the pretreated catalyst with an inert gas prior to using the conditioned catalyst to produce olefins. Without wishing to be bound by any particular theory, it is believed that the hydrocarbon pretreatment procedure described herein results in a deposition, on the exterior surface of, and around to entrance to, the zeolite pores, of what is believed to be a coke precursor consisting of a polymeric carbonaceous material having a high hydrogen to carbon ratio, in very small amounts. A zeolite having a coke precursor derived from methanol deposited thereon absent the heat conditioning step performs substantially the same as the untreated zeolite. It is concluded from this observation that the acid sites on which the coke precursor are deposited are available for catalytic reaction and the zeolite still possesses an active surface. However, it has also been found that the heat conditioning step described herein changes the nature of the coke precursor, and the extent and effect of this change is a function of the heat conditioning temperature. More specifically, it is believed that the coke precursor can be selectively converted to coke in a conventional sense (e.g. low hydrogen to carbon ratio), with the degree of this conversion being disproportionate relative to the strong and weaker acid sites when the heat conditioning step is controlled as described herein. In this manner a much greater degree of coke formation is believed to be achieved on the strong acid sites relative to the weaker acid sites. Thus, the heat conditioning step of the present invention is believed to selectively deactivate the strong acid sites of the zeolite surface to a much greater extent than the weaker acid sites. Apparently, very little deactivation of the essential weaker acid sites occurs during the 2 step treatment of the present invention. This selective deactivation of strong acid sites is significant because it is the strong acid sites which are believed to be primarily responsible for subsequent coke formation, as well as the undesired by-products, produced during the hydrocarbon conversion process, while it is the weaker acid sites which are responsible for the desired catalytic effect. The overall effect of the 2 step treatment is to substantially increase catalyst life while improving, or at least maintaining, olefin selectivity. If the heat conditioning temperature is too high, the selectivity in the deactivation of the acid sites is lost and substantial drop in desired catalytic activity is observed.
Accordingly, in one aspect of the present invention there is provided a process for modifying a crystalline metalosilicate shape selective acidic zeolite free of coke deposits thereon, wherein said metal of the metalosilicate is selected from the group consisting of aluminum, gallium, and mixtures thereof, to improve the catalyst life thereof when said modified zeolite is used to convert, a hydrocarbon feed comprising at least one member selected from the group consisting of monohydric alcohols having from 1 to about 4 carbon atoms, ethers derived from said alcohols, and mixtures thereof, to an olefin containing product at a conversion reaction temperature of from about 300.degree. to about 350.degree. C., which zeolite, prior to said modification, is: (a) capable of catalyzing said hydrocarbon conversion reaction; (b) has a channel size and structure such as to permit (i) entry of the hydrocarbon feed into the zeolite and diffusion to the active acid sites within the zeolite, and (ii) formation of olefin products within said zeolite and diffusion of said products out of the zeolite; and (c) hydrothermally stable at a temperature of at least 290.degree. C.; which process comprises:
(A) contacting said zeolite with a coke precursor forming hydrocarbon feed for a period of from about 0.5 to about 4 hours, said coke precursor forming hydrocarbon feed being (i) comprised of at least one member selected from the group consisting of monohydric alcohols having from 1 to about 4 carbon atoms, ethers derived from said alcohols, olefins having from about 1 to about 5 carbon atoms, and mixtures thereof; and (ii) heated to a temperature of from about 320.degree. to about 350.degree. C.; and
(B) contacting the zeolite treated in accordance with Step (A), with an inert gas heated to a temperature of from about 350.degree. to about 425.degree. C. for a period of at least 1 hour.
In another aspect of the present invention, there is provided a process for employing the aforedescribed modified zeolite in the synthesis of olefins.